Tunable attenuators are ubiquitous in fiber optic systems, as a result of the wide variety of applications they enable. In particular, tunable attenuators may be used to dynamically alter the input power at an amplifier node of an optical communication system, where this functionality is necessary to maintain constant gain and low noise figures. The desired time scales for such variable optical attenuators ranges from minutes to sub-microseconds. Alternatively, devices that can provide rapid attenuation changes in a fiber optic line (at rates of, for example, sub-microseconds to picoseconds) may be utilized to realize high speed modulators for encoding data in a communication system. On the other hand, devices whose attenuation changes in response to varying ambient conditions (such as, for example, outside temperature, pressure, chemical compositions, etc.) are of immense importance to sensor technologies employing fiber optic detection schemes. In particular, devices with high sensitivity to the ambient refractive index of chemical compositions would be attractive for realizing sensors used for identifying trace gases or chemicals in the atmosphere.
Long period fiber gratings (LPGs) are mode conversion devices that have been used extensively to provide components that offer wavelength-selective attenuation in a wavelength division multiplexed (WDM) communication system. Dynamic tuning of the spectral characteristics of LPGs has been achieved by a variety of techniques. For example, LPGs that couple the core mode to a cladding mode can be tuned dynamically by modulating the refractive index of an outer or inner cladding material that is interrogated by a cladding mode of the fiber. The refractive index of such cladding materials can be varied by temperature, the electro-optic effect or some nonlinear optical effect, depending on the nature of the cladding material used. Alternatively, the LPGs may be mechanically strained to change the refractive index, using, for example, piezo-electric packages, simple motion control housings or magnetically latchable materials. The tuning mechanisms described above serve to shift the spectral response of LPGs from one center wavelength to another. While these techniques are useful for tuning the wavelength-selective attenuation in a fiber optic system, they are not sensitive enough to enable detection of small ambient changes in refractive index due to trace gases in the ambient environment, or to provide high speed modulation (on the order of 100 Ghz or higher). Typically, an ambient refractive index change (Δn) of at least 0.01 is required to achieve “complete” tuning (“complete” being defined as a tunable device that changes state from fully transparent to >20 dB attenuation). To date, means of increasing the grating sensitivity have concentrated on etching fibers down to very small outer diameters (OD) to obtain a cladding mode that is significantly more sensitive to ambient index changes. The highest sensitivity reported to date is on the order of 2×10−3, from fibers etched to an outer diameter of 32 μm. Since trace elements typically change ambient indices by less than a factor of 10−4, and high speed electro-optic elements yield index changes on the order of 5×10−4, the etching fiber arrangement remains inadequate. Additionally, a fiber with an outer diameter of only 32 μm would be significantly unreliable as a result of the diminished strength of thin fibers.
In addition, the tuning that is most desirable for dynamic filters is tuning of the strength (loss) of the coupling, not tuning of the resonant wavelength itself. The prior art is replete with “tuning” arrangements that alter the resonant wavelength, but very little has been reported on providing the ability to tune the strength of the coupled signal. One prior art article by V. Grubsky et al., entitled “Long-period fiber gratings with variable coupling for real-time sensing applications”, appearing in Optics Letters, Vol. 25, p. 203 (2000), discloses an arrangement for broadening the bandwidth of LPGs by coupling to a higher-order cladding mode. In this arrangement, greater than 50 nm coupling was achieved, albeit with weak coupling strengths. Grubsky et al. provided the coupling strength tuning using either ambient temperature or applied mechanical strain. While this arrangement achieved coupling strength tuning, the sensitivity of the response is considered to be too low for use in high speed modulation or trace element detection schemes. In the Grubsky et al. arrangement, the sensitivity was fixed by the cladding mode order chosen to induce coupling. Moreover, the wavelength of coupling was constrained by the specific cladding mode that afforded tunable attenuation.
Thus, there exists a remaining need in the art for a fiber grating device that can offer strong tunable attenuation, preferably over bandwidths exceeding 20 nm, whose coupling strength is tuned by small changes in the refractive index of the active material. The active material may either be a coating whose index changes are due to incorporation of trace elements in the atmosphere, or an electro-optic or nonlinear-optic material that changes index as a result of an applied electric field or optical radiation. A practical device would also allow operation at any desired wavelength without impacting the sensitivity, since the desired wavelength of operation would be different for various applications.